The applicant has previously proposed a variety of forms of tamper resistant enclosures, such as those described in U.S. Pat. No. 5,858,500, and UK Patent Applications 2220513, 2258075, 2256956, 2256957, 2256958 and 2270785, the disclosures of which are incorporated herein by reference.
These enclosures are in the form of envelopes and shallow boxes, the walls of which are formed by folding flexible sheets incorporating tamper detection features. Included in such sheets are layers of flexible material including a matrix of semi-conductive lines printed on thin insulating film. The matrix of lines forms a continuous conductor which is broken if attempts are made to penetrate the film. The circuit is monitored by opening the conductor at one point and measuring the resistance between the two ends of the circuit. The sheets are folded and overlapped to create an enclosure of wedge-shaped, cuboid or cube form, such as disclosed in GB 2 258 075 A in which a laminate is folded about a plurality of fold lines to form an enclosure. In U.S. Pat. No. 5,858,500 the envelope or box formed of the flexible sheet is potted in a settable material.
The enclosures are intended to surround an item to be protected, such as an electronic device, which may be an encryption module, chip or other circuit for processing, containing or carrying potentially valuable information. As noted above, any attempt to penetrate the enclosure results in damage to one or more of the lines, and this damage is detectable as a change in an electrical characteristic of the conductor. On detection of such a change the valuable information contained within the item is typically erased or destroyed and an alarm may be activated.
It may however be relatively time consuming and expensive to enclose and surround an item in such an enclosure. Also, the provision of an enclosure which completely surrounds an item places restrictions on the manner in which the item may be located and positioned within a larger device and is likely to, for example, preclude conventional surface mounting.
US Patent Application Publication No. US2002/0002683 A1 to Benson et. al. discloses a Security Module System comprising a cover which encloses the components to be protected and abuts the substrate, on which the said components are mounted, using a ball grid array connection system. The cover comprises a serpentine pattern of metallic conductors which may be interconnected with a pattern of metallic conductors embedded in the substrate by a system of plated through-holes and blind vias to form a three dimensional array of conductors surrounding the components being protected. To prevent the pattern and location of the metallic conductors being detected by non-destructive techniques such as X-rays, back planes of X-ray opaque material are laminated into the cover and the substrate.
Furthermore, to deter chemical attack on the system additional elements such as conductive ink fuses are provided on the substrate.
The system disclosed has a number of shortcomings in relation to resistance to intrusive attacks, for example the X-ray opaque backplane can be easily located and could be ground away or electrochemically etched to allow the underlying pattern of metallic conductors to be exposed by X-ray or other non-destructive techniques. Areas of the metallic conductors could then be effectively bridged by attaching wire links to the serpentine pattern of conductors and the cover or substrate penetrated without triggering the tamper respondent circuit.
Similarly, the vias in the side wall of the cover could be located and similarly bridged without triggering the tamper respondent circuit. The side walls are not protected by the pattern of serpentine conductors and therefore present an area susceptible to attack, as does the system of ball grid array interconnections.
It would also be possible by similar techniques to locate the positions of the conductive ink fuses on the circuit board and to direct a chemical attack at locations away from the fuses without triggering a tamper response.
The system disclosed teaches the protection of a complete board of components. In some cases it may be desirable to protect a small area of a board containing only one or a few items and this is not addressed by the referenced application. Indeed, given the complexity of the approach used it is unlikely that that this system could be effectively used to protect such small areas.
Preferred embodiments of the current invention are designed to address the aforementioned shortcomings.